Reincarnation is to be made flesh again. The belief that one comes from spirit into physical body more than one time during the course of eternity. A theory of where past lives come from.
Reincarnation is ultimately the path to enlightenment and pureness of being. There are many lessons to learn before one can ascend to the highest plane, and usually those lessons take more than one life. If a person has earned enough good karma during their life, he/she will ascend in the next life as a higher being. If the person did not earn enough karma, or earned bad karma, he/she will regress to a lower form in the next life. This process is repeated until the person has mastered all the skills and knowledge set before them and can quite literally learn no more.